Field of the Invention
This invention relates generally to a perpendicular magnetic recording medium for use as a heat-assisted magnetic recording (HAMR) medium, and more particularly to a HAMR medium with an intermediate layer that provides an improved thermal barrier and/or seed layer for the magnetic layer.
Description of the Related Art
In conventional continuous magnetic recording media, the magnetic recording layer is a continuous layer over the entire surface of the disk. In magnetic recording disk drives the magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data regions that define the data “bits” are written precisely and retain their magnetization state until written over by new data bits. As the areal data density (the number of bits that can be recorded on a unit surface area of the disk) increases, the magnetic grains that make up the data bits can be so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, media with high magneto-crystalline anisotropy (Ku) are required. The thermal stability of a magnetic grain is to a large extent determined by KuV, where V is the volume of the magnetic grain. Thus a recording layer with a high Ku is important for thermal stability. However, increasing Ku also increases the coercivity of the media, which can exceed the write field capability of the write head.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is heat-assisted magnetic recording (HAMR), wherein the magnetic recording material is heated locally during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating temperature range of approximately 15-60° C.). In some proposed HAMR systems, the magnetic recording material is heated to near or above its Curie temperature. The recorded data is then read back at ambient temperature by a conventional magnetoresistive read head.
The most common type of proposed HAMR disk drive uses a laser source and an optical waveguide with a near-field transducer (NFT). A “near-field” transducer refers to “near-field optics”, wherein the passage of light is through an element with sub-wavelength features and the light is coupled to a second element, such as a substrate like a magnetic recording medium, located a sub-wavelength distance from the first element. The NFT is typically located at the air-bearing surface (ABS) of the air-bearing slider that also supports the read/write head and rides or “flies” above the disk surface.
One type of proposed high-Ku HAMR media with perpendicular magnetic anisotropy is an alloy of FePt (or CoPt) alloy chemically-ordered in the L10 phase. The chemically-ordered FePt alloy, in its bulk form, is known as a face-centered tetragonal (FCT) L10-ordered phase material (also called a CuAu material). The c-axis of the L10 phase is the easy axis of magnetization and is oriented perpendicular to the disk substrate. The FePt alloy requires deposition at high temperature or subsequent high-temperature annealing to achieve the desired chemical ordering to the L10 phase.
The FePt alloy magnetic layer also typically includes a segregant like C, SiO2, TiO2, TaOx, ZrO2, SiC, SiN, TiC, TiN B, BC or BN that forms between the FePt grains and reduces the grain size. The use of carbon (C) has been proposed as a segregant for the FePt grains in HAMR media. To obtain the required microstructure and magnetic properties, the FePt needs to be deposited with the substrate maintained at high temperatures (e.g., about 500 to 700° C.). In published patent application US 20130114165 A1, titled “FePt—C BASED MAGNETIC RECORDING MEDIA WITH ONION-LIKE CARBON PROTECTION LAYER” and assigned to the same assignee as this application, the C segregant is described as shells of multiple graphitic carbon layers that encapsulate the FePt grains, which then have a generally spherical shape.
In HAMR media, a MgO seed layer is used to induce the desirable (001) texture to the FePt magnetic grains and influence their geometrical microstructure. However, the use of MgO as a seed layer causes multiple problems. Corrosion of the media with Mg migration to the surface of the media can cause severe head-disk interface issues, primarily due to the sensitivity/reactivity of MgO to moisture in the air. MgO is also known to have a low sputtering rate and can generate undesirable particles during radio frequency (RF) sputtering. Also, MgO has moderate thermal conductivity that limits the thermal barrier performance of the MgO layer. This can necessitate an additional barrier layer with lower thermal conductivity to keep laser power of the write head low, so that the write head remains stable with good performance over its anticipated life time and does not degrade.
What is needed is an alternative material having superior performance to MgO to not only mitigate the problems caused by MgO but also to provide a better seed and thermal barrier layer for HAMR media.